204 



Even if we cannot yet explain with certainty the meclianics of wilting 

 which take place here (the explanation given by W. Wolf^ does not 

 seem to be sufficient) we will, however, scarcely go astray in assuming 

 that, as the result of the excessive accumulation of carbon dioxid in the soil 

 water, the normal elimination by the roots of carbon dioxid, which is con- 

 siderable in vigorously growing plants, is at once arrested. An unusually 

 high gas pressure must therefore be produced within the plant, increasing 

 to a positive pressure in the ducts and reducing their ability to conduct 

 water to the aerial parts. The power of the ducts to conduct water will be 

 decreased by the amount taken up by the negative pressure in the ducts. If 

 thereby this conduction of water is weakened without corresponding re- 

 duction of the use of water in the leaves, wilting results immediately. If 

 the plants are placed in distilled water, as in Wolf's experiments, a normal 

 appearance and normal functions again set in. The distilled water in this 

 case is like a sponge, absorbing the carbon dioxid and other excretory pro- 

 ducts of the roots. 



Finally the result is the same for the root, whether the carbon dioxid 

 appears dissolved in water, or as a gas resulting fi-om an insufficient soil 

 absorption. For the aerial parts of the plant, however, conditions are differ- 

 ent and it is very important whether they come in contact with water rich 

 in carbon dioxid or in air containing the gas. At least Bohm's experiments- 

 on the leaves of green land plants have emphasized this. He immersed 

 leaves of different land plants under water containing carbon dioxid and 

 found that the plant no longer gave off oxygen if the part concerned was 

 prevented from surrounding 'itself with an atmosphere containing carbon 

 dioxid which would cut it off from direct contact with the water. 



The results of excessive watering in pots with the drainage stopped 

 and the consequent cessation of plant and soil activity are best determined 

 by a microscopic comparison with the soil in a pot containing a healthy 

 growing plant. What intense activity is found in the soil ! From the upper 

 surface down to the bottom of the pot (in leaf and heath earth) are found 

 fragments of leaves and stems, on which many kinds of the so-called mold 

 forms with sterile mycelia, or with mature conidia, exercise their power of 

 decomposition. According to the nature of the vegetable matter, Sepedon- 

 ium (chrysospermumf ) , Verticillium ruherrimum, or Penicillium glaucum, 

 Acremonium, Acrocylindrium, Cladosporium penicillioides, different kinds 

 of Fusiarium and many others are found. On the upper surface often still 

 other genera occur, especially the aerobic ones together with living diatoms 

 and other forms of algae. The schizomycetes go deepest of all. Starch 

 granules and bits of cytoplasm are found surrounded by colonies of rod 

 bacteria radially arranged; colonies of bacteria have often been established 

 also on fragments of crystals. All this active life is engaged in reducing 

 the plant substance and favors the processes requiring oxygen, which we 



Jahresber. f. Agrik.-Chemie, 1870-72, II, p. 134. 



Anzeigen der Wien. Akad. d. Wiss., 1872, Nos. 24-25, p. 163. 



